In recent years, for weight reduction of automobiles, an effort has been made to reduce the thickness of steel sheets to be used, through an increase in strength of the steel sheets. As a technique for subjecting difficult-to-form material such as a high strength steel sheet to press forming, a hot forming technique such as a hot press has become more and more adopted in which a material that is to be subjected to forming is preheated before forming.
The hot forming method is advantageous in that forming is carried out at an elevated temperature that renders deformation resistance low and hardening can be carried out simultaneously with the forming. Accordingly, the hot forming method is an excellent forming method that can simultaneously ensure strengthening of the member and the formability. The hot forming method, however, requires heating of a steel sheet to a high temperature of 700° C. or above before forming and thus poses a problem of oxidation of the steel sheet surface. Scale formed of iron oxide produced as a result of the oxidation of the steel sheet surface disadvantageously comes off during pressing, is adhered to a mold, and results in lowered productivity, or the scale stays in products after pressing and leads to poor appearance. Further, when such scale stays, the adhesion between the steel sheet and the coating film is so low that the corrosion resistance is lowered in coating in the next step. Accordingly, after pressing, descaling such as shot blasting is necessary.
In order to solve the above problems, the use of galvanized steel sheets coated with a zinc-based plating or an aluminum-based plating has been proposed as a material for hot forming with a view to inhibiting the oxidation of the base-material steel surface and/or improving the corrosion resistance of press formed products. For example, Patent Literatures 1 and 2 use galvanized steel sheets in hot forming.
Further, Patent Literature 3 proposes a steel sheet that can simplify or eliminate the step of separating the oxide of the formed product surface through an improved adhesion of an oxide film formed in hot forming by specifying the concentration of C, Si, P, and/or Ti in steel and specifying the coverage of Zn on a steel sheet surface and the concentration of Al in a film.